Apparatus and method of regulating the speed of a brushless DC motor

ABSTRACT

A control circuit is provided for regulating the rotational speed of a brushless DC motor by pulse width modulating at least one power transistor to pass a motor supply signal to the motor armature. A voltage averaging circuit generates an averaged signal indicative of the average voltage level being supplied to the motor. The averaged voltage signal is compared against a reference voltage to determine motor speed error in order to maintain the rotational speed of the motor at a generally constant level. A sawtooth or other periodic ramp signal is added to a motor current signal, and this composite signal is monitored by a comparator until it overcomes the motor speed error signal. The pwm circuit thereby modulates the power supply to regulate motor speed while maintaining a symmetrical motor armature current waveform. The control circuit takes advantage of the inherent inductance of the motor windings and the moment of inertia of the rotor assembly as filters to help smooth the physical operation of the motor and to further maintain its desired rotational speed.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 09/131,046, filed Aug. 7, 1998, co-pendingherewith, and incorporated by reference herein.

FIELD OF THE INVENTION

[0002] The present invention relates generally to a motor controller,and more particularly to a controller for controlling the rotationalspeed and armature current of a brushless DC motor.

BACKGROUND OF THE INVENTION

[0003] Control circuits are known for controlling brushless DC motors,such as, for example, regulating the rotational speed of brushless DCfan motors that cool the interiors of computers. One problem withbrushless DC fan motors is that they traditionally have had a narrowusable input range. Fan speed and input current are approximatelyproportional to input voltages. Thus, if the input voltage from anunregulated source such as a battery were used to power a brushless DCfan, such as a typical 24 volt nominal battery, the voltage would varyfrom about 28 volts in float state to about 21 volts in dischargedstate. This change would cause a brushless DC fan rated at a nominal3500 RPM to vary as much as about 1000 RPM over the above-mentionedrange of battery voltages. Such a large variation in RPM means that thefan is not properly cooling a computer at the low-end of the RPM range,and that power is being wasted at the high-end of the RPM range.

[0004] Some brushless DC fan users have multiple input source voltagesthat their equipment is expected to operate from, with 24 volt and 48volt systems being the most common. Such multiple source voltages posethe same problem in resultant RPM variation in a brushless DC fan motoras does a single input voltage source whose voltage level varies widely.Accordingly, there is a need to provide a brushless DC fan motor havinga high input range with relatively little variation in motor rotationalspeed. For example, in the telecommunications industry, there is a needto provide a brushless DC fan motor having an input range of about 20-60volts with little variation in motor rotational speed. However, otherinput voltage ranges may be provided for other motor applications.

[0005] Linear regulators have been used to regulate brushless DC fanrotational speed. However, the linear regulator approach poses anefficiency problem. A brushless DC fan that draws 18 watts at 21 voltswill draw almost 27 watts when operating at 28 volts, and 54 watts at 56volts input, with the increase in power draw having to be dissipated asheat.

[0006] Pulse width modulation (“pwm”) has also been used in the priorart to regulate motor speed. One method commonly used is to pulse widthmodulate the commutation transistors to the brushless DC motor. Thisemployment of pulse width modulation reduces the dissipation of energyinvolved with changing motor speed. However, pulse width modulating thecommutation transistors does not permit large changes in input voltagewithout widely varying the rotational speed of the brushless DC motor.This method is most commonly used in thermal brushless DC fans to reducebrushless DC fan speed at low temperatures. The speed variation isunfortunately even wider than that of the non-speed controlled type, andclamp dissipation is still relatively high.

[0007] Another pwm approach is to use a fill bridge driver. Thisinvolves placing a bipolar motor winding between the legs of fourswitching transistors and controlling the timing of the pwm modulatorand commutation logic to regulate motor current. Wide input voltageranges are possible with high efficiency. A well designed full bridgedriver can regulate motor speed over a better than 3:1 range of inputvoltage. The chief drawbacks are complicated logic and the difficultiesof driving the four switching transistors without cross conductionthrough the series connected pairs. Although many manufacturers offerintegrated full bridge devices, most suffer from a limitation of currentand/or voltage.

[0008] Another approach is to employ a pwm switching voltage regulatorto accommodate a wide range of input voltages without widely varying therotational speed of the motor. However, this requires relatively bulkyfilter inductors and capacitors.

[0009] Of the above-mentioned pwm approaches, the pwm voltage regulatorregulates motor voltage. The other methods typically regulate motorcurrent. Voltage regulation is preferred to minimize variations indesired brushless DC motor speed. In other words, the variation in motorspeed from motor to motor for a given current is greater than thevariation in motor speed for a given voltage. Additionally, motor torqueis a function of motor current.

[0010] Therefore, if motor current is reduced in order to reduce motorspeed to a low value, the motor torque becomes low. This means that themotor speed is sensitive to applied load (i.e., back pressure). Thissensitivity to back pressure results in large speed deviations from thedesired value. Motor-starting at low desired speeds is also a problem inthat if the motor current is set too low then the motor will not be ableto overcome the magnetic detents used to position the rotor away fromthe null point. Unfortunately, controlling motor voltage while failingto control motor current to adhere to a symmetrical waveform has thepotential to increase vibration and electrical interference.

[0011] Fans typically use one of two types of two-phase DC brushlessmotors, unipolar or bipolar. The difference between the two types isthat a unipolar motor energizes two opposing poles of the four polesavailable, whereas a bipolar motor will energize all four poles at thesame time, with the coils in quadrature having opposite magneticpolarity. Simply stated the unipolar type uses two pairs of coils withone pair energized and the other pair not energized, with the polesalways energized in the same polarity. The bipolar motor energizes thefour poles at the same time with adjacent poles having oppositepolarity. Rotation of the motor of the unipolar type is accomplished byalternating energized pairs while the bipolar motor changes the polarityof the four poles.

[0012] The bipolar motor has double the output of the unipolar motorbecause all of the copper is utilized and all four poles act upon themagnet. Drive complexity is greater as the direction of current must bereversed rather than just interrupted. In both cases however a problemof asymmetrical current in the motor exists. The current in the motorwindings is reversed twice for each complete revolution of the bipolarmotor. Various factors influence or modify the symmetry of the motorsuch as the degree of magnet strength, offset in the position sensor,mechanical variations in the motor components, and variations in wireresistance. This causes the current levels and the waveform shapes todiffer from each other within a rotational period and allow differenttorques to be applied to the rotor, increasing vibration and noise.Accordingly, it would be desirable to provide an apparatus and methodwhich may correct such non-ideal behavior in both unipolar and bipolarmotors.

[0013] It is also an object of the present invention to provide abrushless DC motor regulator which handles a relatively wide range ofinput voltages with little variation in the rotational speed of themotor.

[0014] It is another object of the present invention to provide abrushless DC motor regulator which controls motor armature current to asubstantially symmetrical waveform.

[0015] It is a further object of the present invention to provide abrushless DC motor regulator that eliminates the relatively bulky filtercapacitors and inductors interfacing the regulator and motor.

[0016] The above and other objects and advantages of the presentinvention will become more readily apparent when the following detaileddescription is read in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

[0017] According to one aspect of the present invention a controlcircuit for controlling the rotational speed of a brushless DC motor isprovided. The control circuit includes an electrical conduction switchhaving an input, an output, and a control terminal for passing a motorsupply signal to a brushless DC motor from a voltage across first andsecond terminals of a DC voltage source. The input terminal of theswitch is to be coupled to the first terminal of the DC voltage source,and the output terminal of the switch is to be coupled to the firstterminal of the brushless DC motor. A voltage averaging circuit isprovided having first and second input terminals and an output terminalfor averaging the voltage level of the motor supply signal. The firstinput terminal of the voltage averaging circuit is coupled to the outputof the switch, and the second terminal of the voltage averaging circuitis to be coupled to the second terminal of the voltage source. Adifferential amplifier has first and second input terminals and anoutput terminal for generating a signal corresponding to motor speederror. The first input terminal of the differential amplifier is coupledto a voltage reference potential indicative of the desired motor speed,and the second input terminal of the differential amplifier is coupledto the output terminal of the voltage averaging circuit.

[0018] A pulse width modulator (“pwm”) of the invention has first andsecond input terminals and an output terminal. The first input terminalof the pwm is coupled to the output terminal of the differentialamplifier for receiving the signal corresponding to motor speed error,the second input terminal of the pwm is coupled to a signalcorresponding to the change in motor current, and the output terminal ofthe pwm is coupled to the control terminal of the electrical conductionswitch. The pwm turns the switch on at a periodic rate, and turns theswitch off after a delay, or pulse width, indicative of the differencein voltage level between the signal corresponding to motor speed errorand the signal corresponding to change in motor current, in order toprovide a motor supply signal having a substantially constant averagevoltage level corresponding with the desired motor speed and asubstantially symmetrical current waveform. Preferably, the motorwindings serve as an inductive filter to help smooth changes in current,and the rotor mass of the motor serves to help smooth the rotationalspeed of the motor.

[0019] According to another aspect of the present invention, a controlcircuit for controlling the rotational speed of a brushless DC motor isprovided. The control circuit includes first means to be coupled to anelectrical power source for switchably passing a motor supply signal toa brushless DC motor. A second means is coupled to an output of thefirst means for generating an averaged signal by averaging the voltageof the motor supply signal. A third means is coupled to an output of thefirst means for generating a signal indicative of the change in motorcurrent. A fourth means is coupled to an output of the second means forgenerating a speed error signal having a voltage level indicative of thedifference in voltage between the voltage level of the averaged signalof the second means and a reference voltage. A fifth means turns on thefirst means periodically, and turns off the first means following adelay corresponding to the difference between the value of the speederror signal and the value of the change in motor current signal. Thesemeans provide a substantially constant average motor supply voltagelevel resulting in a substantially constant motor speed approximatelyequal to a desired motor speed, and a substantially symmetrical motorcurrent supply signal waveform.

[0020] According to yet another aspect of the present invention, amethod of controlling the rotational speed of a brushless DC motor isprovided. A motor supply signal is switchably passed from an electricalpower source to a brushless DC motor. The voltage level of the motorsupply signal is averaged to form an averaged signal. An error signal isgenerated having a voltage level indicative of the difference in voltagebetween the averaged signal and a reference voltage. A motor currentsignal is generated having a voltage level indicative of the change incurrent of the motor supply signal. The motor supply signal is modulatedin response to the difference in value between the error signal and themotor current signal in order to provide a substantially constantvoltage level and a substantially symmetrical motor current waveform.

[0021] One advantage of the present invention is that the motor voltagesignal is compared against the reference voltage to generate an errorsignal, and the error signal is in turn compared against the motorcurrent signal to pulse width modulate the motor input signal.Accordingly, the apparatus and method of the present invention employboth a voltage feedback loop, and a current feedback loop embeddedwithin the voltage feedback loop to maintain a substantially constantmotor speed over a wide range of power supply voltages, to accuratelyselect and control motor speed, and to do so while maintaining asubstantially symmetrical armature current waveform.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 illustrates schematically a prior art electrical circuit ofa pwm voltage regulator employing filter capacitors and inductorsinterfacing the regulator to a brushless DC motor.

[0023]FIG. 2 illustrates schematically an electrical circuit of a pwmvoltage regulator for a unipolar motor embodying the present inventionwhich employs the brushless DC motor windings and rotor mass as asubstitute for additional filter inductors and capacitors.

[0024]FIG. 3 illustrates schematically a pwm sub-circuit of the pwmvoltage regulator of FIG. 2.

[0025]FIG. 4 illustrates schematically an alternative currentcompensating pwm sub-circuit embodying the present invention and whichmay form a part of the pwm voltage regulator of FIG. 2.

[0026] FIGS. 5A-5C illustrate three current waveform inputs to a motordemonstrating typical waveform improvements of the pwm sub-circuit ofFIG. 4 when used in the pwm voltage regulator of FIG. 2.

[0027]FIG. 6 illustrates schematically an electrical circuit of a pwmvoltage regulator for a bipolar motor embodying the present inventionand which also employs the brushless DC motor windings and rotor mass asa substitute for additional filter inductors and capacitors.

[0028]FIG. 7 illustrates schematically a current compensating pwmcontrol circuit embodying the present invention for the pwm voltageregulator of FIG. 6.

[0029] FIGS. 8A-8B illustrate two current waveform inputs to a motoroperating at about 2000 RPM demonstrating typical waveform improvementsof the pwm circuit of FIG. 7 when used with the pwm voltage regulator ofFIG. 6.

[0030] FIGS. 9A-9B illustrate two current waveform inputs to a motoroperating at about 3500 RPM demonstrating typical waveform improvementsof the pwm circuit of FIG. 7 when used with the pwm voltage regulator ofFIG. 6.

[0031] FIGS. 10A-10B illustrate two motor winding current waveforms in amotor operating at about 2000 RPM demonstrating typical waveformimprovements of the pwm circuit of FIG. 7 when used with the pwm voltageregulator of FIG. 6.

[0032] FIGS. 11A-11B illustrate two motor winding current waveforms in amotor operating at about 3500 RPM demonstrating typical waveformimprovements of the pwm circuit of FIG. 7 when used with the pwm voltageregulator of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] A prior art pwm voltage regulator will first be explained asbackground to the pwm voltage regulator embodying the present invention.Referring now to the prior art of FIG. 1, a pwm voltage regulator isindicated generally by the reference number 10 and is employed tocontrol the rotational speed of a brushless DC motor 12 enclosed bydashed lines. The regulator 10 includes a positive terminal 11 and anegative terminal 13 for receiving a regulator DC input voltage from apower source (not shown). The pwm voltage regulator 10 includes an inputfilter 2(capacitor 14, a pwm power transistor 16 that is switched on andoff by a pwm modulator 18, a catch diode 20, and an output filtercircuit 22 enclosed by dashed lines which includes an output filterinductor 24 and an output filter capacitor 26. The output filterinductor 24 and the output filter capacitor 26 are typically ratherbulky, thereby imposing design constraints in relation to the increasingdemand for smaller voltage regulators that are either separate from orincorporated in brushless DC motors. The demand for smaller regulatedmotors is particularly high in the computer industry which uses DC fanunits incorporating regulated brushless DC motors for cooling electroniccomponents.

[0034] The output filter circuit 22 smoothes a pwm waveform generated bythe pwm modulator 18 and the switching transistor 16 into a motor supplysignal having an average DC voltage level. This average DC voltage levelof the DC motor input signal determines the rotational speed of themotor 12. In order to maintain the rotational speed of the brushless DCmotor at a generally constant revolutions per minute (rpm), feedback isprovided to the regulator 10. To provide feedback, the voltage level ofthe DC motor input signal is received at the inverting input of adifferential or error amplifier 28 and compared with a reference voltageV_(ref) which is provided at the non-inverting input of the amplifier28. The output of the error amplifier 28 is the difference between thetwo inputs to the error amplifier 28, and is multiplied by the gain ofthe error amplifier. This error voltage output by the error amplifier 28is provided as a feedback signal to the pwm modulator 18 which adjuststhe width of the pulse or modulator signal generated by the pwmmodulator. The modulator signal adjusts the switching on and off time ofthe transistor 16 in order to modulate the regulator input signal, whichin turn adjusts the average voltage level of the motor input signalafter being smoothed by the filter circuit 22, in order to compensatefor deviations in the motor input voltage level sensed by the feedbackcircuitry. The compensation thus tends to maintain the voltage level ofthe motor input signal constant despite changes in the voltage level ofthe regulator input signal or changes to the load in order that therotational speed of the motor 12 remains relatively constant. Aspreviously mentioned, a drawback with the prior art circuit is that theoutput filter inductor and capacitor 24, 26 are relatively bulky andtherefore require considerable mounting space in the regulatorcircuitry. This large space requirement hampers the growing demand toincorporate brushless DC motors and regulator circuitry in ever smallerspaces, such as the relatively small spaces allotted for regulated DCcooling fan units within portable computers.

[0035] Turning now to FIG. 2, a pwm voltage regulator circuit 100 isemployed in a center tap modulation approach for regulating therotational speed of a brushless DC motor 102 enclosed by dashed lines.The motor 102 is a conventional brushless DC motor which may be coupledto a fan 104 used to cool a surrounding area such as the inside of acomputer. The motor 102 includes first and second directional windings106, 108, respectively. Each of the windings 106, 108 has a first endcoupled to an input terminal or center tap 110 of the motor. A secondend 112 of the first winding 106 is coupled to ground potential via afirst commutation switching transistor 114, and a second end 116 of thesecond winding 108 is likewise coupled to ground via a secondcommutation switching transistor 118. The switching transistors 114, 118are alternately turned on and off by means of a conventional commutatorlogic circuit 120. A rotor 122 is caused to rotate, and in turn rotatethe fan 104 coupled thereto, by interacting with an electromagneticfield generated by commutated current flowing through the first andsecond windings 106,108.

[0036] The voltage regulator circuit 100 includes a positive inputterminal 124 and a negative input terminal 126 for receiving thereacrossa DC regulator input signal from a power source (not shown). An inputcapacitor 128 is coupled across the positive and negative inputterminals 124 and 126. Means for switchably passing a motor supplysignal to the brushless DC motor 102 includes, for example, a pwm poweror switch or transistor 130, such as an npn bipolar junction transistor(BJT). The transistor 130 has its collector 132 coupled to the positiveinput terminal 124 and its emitter 134 coupled to an input voltageterminal of the motor 102 at 110. A catch diode 135 has its cathodecoupled to the emitter 134 of the transistor 130 and its anode coupledto ground potential. Means for averaging the voltage of the motor supplysignal includes a series connected resistor 136 and capacitor 138 whichcooperate to form a voltage integrator. The resistor 136 and thecapacitor 138 are coupled between the emitter 134 of the transistor 130and the negative input terminal 126. More specifically, the resistor 136has respective first and second terminals 139, 140, and the capacitor138 has respective first and second terminals 141, 142. The firstterminal 139 of the resistor 136 is coupled to the emitter 134 of thetransistor 130. The second terminal 140 of the resistor 136 is coupledto the first terminal 141 of the capacitor 138 at a junction 144 wherean averaged signal indicative of the average voltage level of the motorsupply signal is generated, and the second terminal 142 of the capacitor138 is coupled to the negative input terminal 126.

[0037] Means for generating a differential signal having a voltage levelindicative of the difference between the voltage level of the averagedsignal and a reference voltage includes a high gain operational or erroramplifier 146, such as a differential voltage amplifier. The erroramplifier 146 has its inverting input coupled to the junction 144 via aresistor 147. The gain of the error amplifier 146 is preferably selectedso that only millivolts of difference between the inverting andnon-inverting inputs will drive the amplifier output to its extreme. Aresistor 149 is coupled between the inverting input of the erroramplifier 146 and ground potential. The non-inverting input of the erroramplifier 146, which is fed the reference voltage, is coupled to avoltage V_(cc) via a resistor 151. The non-inverting input is alsocoupled to ground via a series connected resistor 153 and potentiometer155. An output 148 of the error amplifier 146 is coupled to a pulsewidth modulator (pwm) 150 at an input 152. The pwm 150 is a means forturning on and off the transistor 130 to modulate the motor supplysignal so that the motor supply signal is maintained at a substantiallyconstant voltage level and, in turn, the motor 102 is maintained at asubstantially constant rotational speed. A roll-off capacitor 157 iscoupled between the inverting input and the output 148 of the erroramplifier 146. An output 154 of the pwm 150 is coupled to a base 156 ofthe transistor 130.

[0038] One preferred embodiment of the pulse width modulator 150 isillustrated in FIG. 3. The pwm 150 includes an oscillator sub-circuit200 and a driver sub-circuit 300 each enclosed by dashed lines. Theoscillator 200 includes a comparator 202 having its non-inverting inputcoupled to a V_(cc) source at 204 via a resistor 206. An output 208 ofthe comparator 202 is coupled to its non-inverting input via a resistor210. A resistor 212 is coupled between the non-inverting input of thecomparator 202 and ground potential. The resistors 206, 210 and 212 arecoupled to one another at a junction 214. A timing resistor 216 iscoupled between the output 208 of the comparator 202 and the invertinginput of the comparator. A timing capacitor 218 is coupled between theinverting input of the comparator 202 and ground potential.

[0039] The driver sub-circuit 300 includes a comparator 302 having itsnon-inverting input coupled to the inverting input of the comparator 202of the oscillator sub-circuit 200. The inverting input of the comparator302 at terminal 304 receives the error voltage signal from the output148 of the error amplifier 146 shown in FIG. 2. An output 306 of thecomparator 302 is coupled to a base of a transistor 308 via a resistor314. The transistor 308, which serves as a pwm driver transistor, isshown as an npn BJT, but may be an FET or other suitable transistor fordriving the pulse width modulator. An emitter of the transistor 308 iscoupled to ground potential, and a collector of the transistor 308 iscoupled at its output 310 to the base 156 of the power transistor 130shown in FIG. 2.

[0040] Referring now to the operation of the pwm voltage regulatorcircuit 100 shown in FIG. 2, the regulator circuit 100 receives a DCregulator input signal across the positive and negative input terminals124, 126 from a power source (not shown) such as a DC power supply or anAC source that is rectified into DC voltage. The DC regulator inputsignal is initially filtered by the input capacitor 128 to furthersmooth the input voltage signal and to remove any unwanted transientvoltage fluctuations. The motor supply signal derived from the DC powersupply is modulated by the combination of the pwm 150 and the transistor130 to generate a motor supply signal having a predetermined averagevoltage level suitable for operating the motor 102 at a desiredrotational speed. Because the rotational speed of the motor 102 is afunction of the voltage level of the motor supply signal, it isimportant to maintain this voltage level at substantially the samevalue.

[0041] The pwm 150 sends a modulator signal to the base 156 of theswitching transistor 130 to modulate the motor supply signal. Themodulated motor supply signal present at the emitter 134 of thetransistor 130 is the signal used to regulate the rotational speed ofthe motor 102. The voltage level of the modulated motor supply signal isaveraged at the junction 144 by the series combination of the resistor136 and the capacitor 138 to form an averaged signal, and is thusindicative of the average voltage level of the motor supply signal. Thevoltage level of the averaged signal is a function of the pulse width ofthe modulated motor supply signal.

[0042] The voltage level of this averaged signal at the junction 144 isreduced by the resistors 147, 149, and this reduced voltage level of theaveraged signal is received at the inverting input of the erroramplifier 146 and compared with a reference, such as V_(ref), togenerate a differential or error signal at the output 148 of the erroramplifier 146. V_(ref) is determined by the resistors 151, 153 andadjusted by the potentiometer 155. The reference voltage V_(ref) is afixed voltage level which is compared with the reduced voltage level ofthe averaged signal to determine if there is any deviation in thedifference between the voltage level of V_(ref) and the reduced voltageof the averaged signal representing the motor supply signal or voltage,and thus indicating a tendency for the rotational speed of the motor 102to change or drift over changes in input voltage to the pwm voltageregulator circuit 100. As an example, the voltage level of the averagedsignal V_(m) is selected as 12.75 volts, V_(ref) is 0.25 volt and theresistors 147, 149 are selected to reduce the voltage level of theaveraged signal by a factor of 50 in order that V_(m)=((resistance ofthe resistor 147/resistance of the resistor149)*V_(ref))+V_(ref)=((50)*0.25)+0.25)=12.75 volts.

[0043] If there is a deviation between the ideal voltage V_(ref) (i.e.,0.25 volt in this example) and that of the voltage level of the reducedaveraged signal at the inverting input of the error amplifier 146, theerror amplifier 146, in order to compensate for any change in thevoltage level of the motor supply signal (i.e., a deviation from 12.75volts in this example), will generate an error signal at the output 148of the amplifier 146 having a voltage magnitude proportional to thedifference between the voltage levels present at the inverting andnon-inverting inputs of the amplifier 146. When the reduced voltagelevel of the averaged signal drops slightly in relation to V_(ref)because of, for example, a load increase or input voltage drop, thevoltage level of the amplifier signal generated at the output 148 of theamplifier 146 will increase slightly. The increased voltage level of theamplifier signal will then be fed to the input 152 of the pwm 150 toslightly increase the duration or pulse width of the modulator signalgenerated at the output 154 of the pwm 150.

[0044] The increased duration of the modulator signal is fed to the base156 of the power transistor 130 to increase the pulse width or durationof the turn-on time of the transistor 130. The increased turn-on timethus increases the pulse width of the modulated motor supply signalpresent at the emitter 134 of the transistor 130 which is fed to theinput voltage terminal 110 of the brushless DC motor 102. The increasedduration or pulse width of the modulated motor supply signal raises theaverage voltage level of the motor supply signal, to compensate for theslight drop in the voltage level of the motor supply signal, therebymaintaining the rotational speed of the motor at a generally constantrpm. Conversely, if the reduced voltage level of the averaged signalincreases slightly, the amplifier 146, the pwm 150 and the transistor130 cooperate in a fashion opposite to that just described to decreasethe pulse width of the motor supply signal for decreasing the averagevoltage level of the motor supply signal. The roll-off capacitor 157 iscoupled across the error amplifier 146 to prevent the output of theerror amplifier from slewing to its limits in response to V_(m) changesby means of reducing the high frequency gain of the error amplifier 146to the point that the pwm 150 can follow the error amplifier output. Thecatch diode 135 prevents the inductor current from decaying at a rapidrate and the voltage at the center tap 100 from falling below groundpotential in order to maintain the average voltage at the center tap asthe motor current is being commutated.

[0045] The pwm voltage regulator circuit just described is known tosubstantially maintain the rotational speed of a brushless DC motor overa wide range of motor supply voltages while also maintaining asymmetrical current waveform. An example of motor rotational speed andcurrent as a function of motor supply voltage is set forth in Table 1.TABLE 1 Voltage Speed (RPM) Current (Amperes) 19 3680 0.82 20 3180 0.8621 3210 0.84 30 3230 0.61 48 3230 0.41 60 3230 0.35

[0046] As can be seen from Table 1, over a motor supply voltage range of20 volts to 60 volts, the rotational speed of a brushless DC motorcontrolled by the regulator circuit of the present invention ismaintained substantially constant (i.e., the rotational speed varies1.5%) as compared with conventional motors. As also shown in the Table,the rotational speed of the motor shows no discernible fluctuation overa motor supply voltage range of 30 to 60 volts.

[0047] An advantage of employing the above-described voltage modecontrol for a brushless DC motor is that the control permits a hightorque for starting the motor and a narrow speed for distribution rangefor tightly regulating the rotational speed of the motor over a largerange of motor supply voltages. A further advantage of applying centertap modulation is that this type of modulation possesses the superiorlinear transfer characteristics found in full bridge modulation withoutcertain drawbacks of full bridge modulation, including: the complexityof logic and sequencing of transistor switches, the possibility of crossconduction, and in turn, short circuiting across the input source,difficulty in sensing average motor coil voltage, sensing continuousmotor current, and the high parts count inherent in employing fullbridge modulation.

[0048] The operation of the pwm 150 of FIG. 2 will be explained morefully with reference to FIG. 3. Preferably, the resistors 206, 210 and212 are selected to be of equal resistance. When the output of thecomparator 202 is low, the junction 214 of the resistors 206, 210 and212 is at {fraction (1/3)} V_(cc). When the output of the comparator 202is high, the junction is at {fraction (2/3)} V_(cc). The timingcapacitor 218 is charged and discharged between {fraction (1/3)} V_(cc)and {fraction (2/3)} V_(cc) by the timing resistor 216. The frequency ofoscillation is primarily a function of the capacitance level of thetiming capacitor 218 and the resistance level of the timing resistor216, and the duty cycle is preferably about 50%. A ramp voltagegenerated by the timing resistor 216 and the timing capacitor 218 isapplied to the non-inverting input of the comparator 302 of the driversub-circuit 300. When the collector of the driver transistor 308 is low,the pwm switch transistor 130, shown in FIG. 2, is off ornon-conducting, and the center tap 110 of the motor 102 is at 0 volts.When the collector of the driver transistor 308 is high, then the pwmswitch transistor 130 is on or conducting, and the center tap 110 of themotor 102 is coupled to V_(in).

[0049] As can be seen in FIG. 2, there is no filter circuit external ofthe motor 102 for smoothing the voltage level of the motor supply signalin order to maintain the rotational speed of the motor 102 at agenerally constant rpm. The regulator 100 uses the windings 106, 108 ofthe motor 102 to integrate the pwm voltage and to function similarly tothe filter inductor 24 of FIG. 1, and changes in current drawn by themotor 102 are smoothed (i.e., the rotational speed of the motor ismaintained substantially constant) by the mass of the rotor 122 in asimilar way as the filter capacitor 26 of FIG. 1 smoothes the voltagelevel of the motor input signal. In other words, the motor inductance issubstituted for the filter inductor 24 of FIG. 1, and the rotor mass orinertia is substituted for the filter capacitor 26 of FIG. 1. Thus, thebulky filter inductor 24 and the filter capacitor 26 of FIG. 1 areeliminated in the embodiment of FIG. 2. As a result, the regulator 100consumes considerably less space than did prior regulators using bulkyfilter inductors and capacitors. Further, a regulated DC motor or DC fanunit that includes the regulator circuit embodying the present inventionalso consumes less space because of the elimination of the additionalbulky filter components. Accordingly, the regulator circuit 100, or a DCmotor or DC fan unit incorporating the regulator embodying the presentinvention overcomes the space constraints that are found in the prior DCmotors or DC fan units using additional filter components.

[0050] Turning to FIG. 4, another preferred embodiment of the pulsewidth modulator is indicated generally by the reference numeral 150′.The pwm 150′ includes a periodic sub-circuit 400 and a driversub-circuit 500 each enclosed by dashed lines. The periodic sub-circuit400 comprises a periodic signal generator in the form of an oscillatorcomprising a comparator 402 and related circuit components. Thecomparator 402 has its non-inverting input coupled to a V_(cc) source at404 via a resistor 406. An output 408 of the comparator 402 is coupledto its non-inverting input via a resistor 410. A resistor 412 is coupledbetween the non-inverting input of the comparator 402 and groundpotential. The resistors 406, 410 and 412 are coupled to one another ata junction 414. A timing resistor 422 is coupled between the output 408of the comparator 402 and the cathode end of a diode 420. The anode endof the diode 420 is coupled to the inverting input of the comparator402. A timing resistor 416 is coupled between the output 408 and theinverting input of the comparator 402. A timing capacitor 418 is coupledbetween the inverting input of the comparator 402 and ground potential.A resistor 424 is coupled between the inverting input of the comparator402 and a summing junction 430. As described in further detail below,the periodic signal generator transmits a ramp signal to the junction432 defining a sawtooth waveform. However, as may be recognized by thoseskilled in the pertinent art based on the teachings herein, the periodicsignal generator may generate any of numerous different periodic or rampsignals suitable for performing the functions described herein.Similarly, the periodic signal generator may take any of numerousdifferent configurations which now or later become known to thoseskilled in the pertinent art for performing the functions of theperiodic signal generator described herein.

[0051] Means for receiving a motor current sample signal are provided byan input terminal 428. The input terminal 428 may be coupled, forexample, to the common source terminal of the motor MOSFETs of FIG. 2.However, as may be recognized by those skilled in the pertinent artbased on the teachings herein, the input terminal 428 may be coupled toany of numerous other motor current sources for generating the motorcurrent signal described herein. A resistor 426 is coupled between theinput terminal 428 and the summing junction 430. Accordingly, thesumming junction 430 provides a signal indicative of the sum of theperiodic or ramp signal received from junction 432 and the motor currentsignal received from the input terminal 428.

[0052] The driver sub-circuit 500 includes a comparator 502 having itsnon-inverting input coupled to the summing junction 430 of the periodicsub-circuit 400. The inverting input of the comparator 502 receives atterminal 504 the error voltage signal from the output 148 of the erroramplifier 146 of FIG. 2. An output 506 of the comparator 502 is coupledto a Reset input of an RS flip-flop 512. The Set input of flip-flop 512is coupled to the output 408 of the periodic sub-circuit 400. Theinverting output of the flip-flop 512 is coupled through a resistor 514to the base of a transistor 508. An emitter of the transistor 508 iscoupled to ground potential, and a collector of the transistor 508 iscoupled at its output 510 to the base 156 of the power transistor 130 ofFIG. 2. The transistor 508, which serves as a pwm driver transistor, isshown as an npn BJT, but may be an FET or other suitable transistor orother electrical conduction switch for driving the pulse width modulatorof the invention. Similarly, as may be recognized by those skilled inthe pertinent art based on the teachings herein, the flip-flop 512 maytake the form of any of numerous binary state or like devices which nowor later become known to those skilled in the pertinent art forperforming the functions of the flip-flop described herein.

[0053] The resistors 406, 410 and 412 may be selected to be of equalresistance. Accordingly, when the output of the comparator 402 is low,the junction 414 of the resistors 406, 410 and 412 is at {fraction(1/3)} V_(cc), When the output of the comparator 402 is high, thejunction 414 is at {fraction (2/3)} V_(cc). The timing capacitor 418 isperiodically charged from {fraction (1/3)} V_(cc) to {fraction (2/3)}V_(cc) by the timing resistor 416. The timing capacitor 418 isperiodically discharged from {fraction (2/3)} V_(cc) to {fraction (1/3)}V_(cc) by the timing resistor 422 through diode 420. The frequency ofoscillation is primarily a function of the capacitance level of thetiming capacitor 418 and the resistance levels of the timing resistors416 and 422. Timing resistor 416 determines the charge period, and theequivalent resistance of parallel resistors 416 and 422 determines thedischarge period. Accordingly, a ramp voltage generated by the timingresistor 416 and the timing capacitor 418 is applied to the junction 432and, in turn, to the summing junction 430. Input terminal 428 passes amotor current signal across resistor 426 to summing junction 430. Thus,the resultant signal at the summing junction 430 is approximately equalto the sum of the ramp signal and the motor current signal, and thesummed signal is coupled to the non-inverting input of the comparator502 of the driver sub-circuit 500. The output terminal 506 of thecomparator 502 is coupled to the Reset input of flip-flop 512, therebycausing the flip-flop 512 to Reset whenever the value of the motorcurrent plus ramp from the summing junction 430 exceeds the value of theerror signal from the input terminal 504 as applied to the invertinginput of the comparator 502. The Set input of the flip-flop 512, on theother hand, is activated every time the output 408 of the comparator 402goes low, thereby activating the inverted output of flip-flop 512 at thestart of each ramp cycle coinciding with the ramp signal received at thesumming junction 430 across the resistor 424. Once activated, theinverted output of the flip-flop 512 drives the collector of the drivertransistor 508 high. When the collector of the driver transistor 508 islow, the pwm switch transistor 130 of FIG. 2 is off or non-conducting,and the center tap 110 of the motor 102 is at approximately 0 volts.When the collector of the driver transistor 508 is high, then the pwmswitch transistor 130 is on or conducting, and the center tap 110 of themotor 102 is coupled to V_(in) in FIG. 2. As may be recognized by thoseskilled in the pertinent art based on the teachings herein, the periodicsignal generator may generate any of numerous different periodic or rampsignals suitable for performing the functions described herein.

[0054]FIG. 5A depicts a typical prior art pwm voltage regulator motorcurrent waveform resulting when motor commutation happens to be ideal.As can be seen, the waveform is symmetrical from pulse to pulse, butnon-symmetrical within each pulse. More frequently however, prior artmotors will exhibit non-ideal commutation with a resultant motor currentwaveform such as that depicted in FIG. 5B. The waveform of FIG. 5B isnon-symmetrical from pulse to pulse in addition to being non-symmetricalwithin each pulse. One advantage of the present invention is that asymmetrical motor current waveform, such as that depicted in FIG. 5C, isattainable via application of the present invention to brushless DCmotors such as those used in the prior art.

[0055] In the operation of the apparatus and method of the invention,the motor is powered by the voltage pulses passing across the pwm switchtransistor 130 of FIG. 2 only when the output 510 of the transistor 508is activated. The transistor 508 is activated periodically when theoutput 408 of the comparator 402 goes low, such activationscorresponding to the start of a periodic sawtooth or other ramp signalgenerated by the oscillator or other periodic signal generator. Theoutput 510 is effectively deactivated whenever the sum of the rampsignal at 432 and the motor current signal at 428 exceed the value ofthe error voltage 504 corresponding to the difference between motoractual speed and desired speed. Thus, motor speed is primarilycontrolled by the circuit of FIG. 2, while motor current is primarilycontrolled by the sub-circuit 400 of FIG. 4. The result is accuratemotor speed control accompanied by symmetrical motor current waveforms,such as depicted in FIG. 5C. A further advantage of the symmetricalmotor current waveform of the present invention is that it may haveattendant acoustical benefits when realized in a fan motor controllerfor lower inertia fan assemblies.

[0056] Turning now to FIG. 6, a pwm voltage regulator circuit 600embodying the invention is employed in an H-bridge modulation approachfor regulating the rotational speed of a bipolar brushless DC motorwinding 622. The motor is a conventional brushless DC motor which may becoupled to a fan used to cool a surrounding area such as the inside of acomputer.

[0057] The voltage regulator circuit 600 includes a positive inputterminal 624 and a negative input terminal 626 for receiving thereacrossa DC regulator input signal from a power source (not shown). An inputcapacitor 628 is coupled across the positive and negative inputterminals 624 and 626. Means for switchably passing a motor supplysignal to a brushless DC motor winding 622 includes, for example, afirst pwm switch or transistor 630, and a second pwm switch ortransistor 631, such as the N-channel MOSFETs illustrated. Thetransistor 630 has its drain coupled to the positive input terminal 624via junction terminal 610, its source coupled to an input voltageterminal of the motor winding 622 at terminal 684, and its gate coupledto a first pwm driver terminal 656. The transistor 631 has its draincoupled to the positive input terminal 624 via junction terminal 610,its source coupled to an input voltage terminal of the motor winding 622at terminal 686, and its gate coupled to a second pwm driver terminal657.

[0058] The motor winding 622 has a first end coupled to a first inputterminal 684, and a second end coupled to a second input terminal 686.The first input terminal 684 is alternately connected to groundpotential via a first commutation switching transistor 614, and thesecond input terminal 686 is likewise alternately connected to groundpotential via a second commutation switching transistor 618. Theswitching transistors 614, 618 are alternately turned on and off bymeans of a Hall Effect sensor 660. The Hall Effect sensor 660 is coupledto a first output terminal 662 and a second output terminal 664. Outputterminal 662 is coupled to the gate of a commutation transistor 614,which has its drain coupled to motor input terminal 684. Output terminal664 is coupled to the gate of a commutation transistor 618, which hasits drain coupled to motor input terminal 686. The source terminals ofcommutation transistors 614 and 618 are coupled together and thencoupled to motor current sample terminal 682. Terminal 682 is droppedacross resistor 680 to negative input terminal 626, which is coupled toground potential. Negative V_(CC) potential is coupled to the anode of adiode 672, the cathode of which is connected to driver power terminal668. Terminal 668 is coupled to bootstrap capacitor 676 which is thencoupled to motor input terminal 684. Negative V_(cc) potential is alsocoupled to the anode of a diode 674, the cathode of which is connectedto driver power terminal 670. Terminal 670 is coupled to bootstrapcapacitor 678 which is then coupled to motor input terminal 686. Themotor winding 622 is caused to rotate by interacting with anelectromagnetic field generated by commutated current flowingtherethrough.

[0059] Turning to FIG. 7, a preferred embodiment of the pulse widthmodulator for a bipolar motor is indicated generally by the referencenumeral 650. The pwm 650 includes a voltage sub-circuit 760, a periodicsub-circuit 770, and a driver sub-circuit 780 each enclosed by dashedlines.

[0060] The voltage sub-circuit 760 comprises means for averaging thevoltage of the motor supply signal including series connected resistors636 and 637, and an averaging capacitor 638 which cooperate to form avoltage averaging circuit. The resistor 636 is coupled between motorwinding terminal 684 and terminal 644, and the resistor 637 is coupledbetween motor winding terminal 686 and junction 644. The averagingcapacitor 638 is then coupled between junction 644 and ground potential.A first voltage dividing resistor 647 is coupled between junction 644and the inverting input of an error amplifier 646. A second voltagedividing resistor 649 is coupled between the inverting input of theerror amplifier 646 and ground potential. The non-inverting input to theerror amplifier 646 is coupled to a reference voltage V_(REF). Theoutput of the error amplifier 646 is coupled to a voltage error terminal652. A capacitor 657 is coupled between the terminal 652 and theinverting input to the error amplifier 646. A resistor 820 is coupledbetween terminal 652 and terminal 804. A resistor 822 is coupled betweenterminal 804 and ground.

[0061] Means for generating a differential signal at terminal 652 havinga voltage level indicative of the difference between the voltage levelof the averaged signal and a reference voltage includes the high gainoperational or error amplifier 646, which in the preferred embodiment ofthe present invention is a differential voltage amplifier. The erroramplifier 646 has its inverting input coupled to the junction 644 via aresistor 647. The gain of the error amplifier 646 is preferably selectedso that only millivolts of difference between the inverting andnon-inverting inputs will drive the amplifier output to its extreme. Aresistor 649 is coupled between the inverting input of the erroramplifier 646 and ground potential. The non-inverting input of the erroramplifier 646 is fed the reference voltage V_(REF). An output terminal652 of the error amplifier 646 is coupled to the driver sub-circuit 780at terminal 804. The pwm 650 provides a means for turning on and off thepwm switching transistors 630 and 631 of FIG. 6 to modulate the motorsupply signal so that the motor supply signal is maintained at asubstantially constant voltage level and, in turn, the motor winding 622is maintained at a substantially constant rotational speed. A roll-offcapacitor 657 is coupled between the inverting input and the output ofthe error amplifier 646.

[0062] The periodic sub-circuit 770 comprises a periodic signalgenerator in the form of an oscillator comprising a comparator 702 andrelated circuit components. The comparator 702 has its non-invertinginput coupled to a V_(cc) source at 704 via a resistor 706. An output708 of the comparator 702 is coupled to its non-inverting input via aresistor 710. A resistor 712 is coupled between the non-inverting inputof the comparator 702 and ground potential. The resistors 706, 710 and712 are coupled to one another at a junction 714. A timing resistor 722is coupled between the output 708 of the comparator 702 and the cathodeend of a diode 720. The anode end of the diode 720 is coupled to theinverting input of the comparator 702. A timing resistor 716 is coupledbetween the output 708 and the inverting input of the comparator 702. Atiming capacitor 718 is coupled between the inverting input of thecomparator 702 and ground potential. A resistor 724 is coupled betweenthe inverting input of the comparator 702 and a summing junction 730. Asdescribed in further detail below, the periodic signal generatortransmits a ramp signal to the junction 732 defining a sawtoothwaveform. However, as may be recognized by those skilled in thepertinent art based on the teachings herein, the periodic signalgenerator may generate any of numerous different periodic or rampsignals suitable for performing the functions described herein.Similarly, the periodic signal generator may take any of numerousdifferent configurations which now or later become known to thoseskilled in the pertinent art for performing the functions of theperiodic signal generator described herein.

[0063] Means for receiving a motor current sample signal are provided byan input terminal 682. The input terminal 682 may be coupled, forexample, to the source terminals of the commutation switch MOSFETs ofFIG. 6. However, as may be recognized by those skilled in the pertinentart based on the teachings herein, the input terminal 682 for receivingthe motor current input signal may be coupled to any of numerous othermotor current sources for generating the motor current signal describedherein. A resistor 726 is coupled between the input terminal 682 and thesumming junction 730. Accordingly, the summing junction 730 provides asignal indicative of the sum of the periodic or ramp signal receivedfrom junction 732 and the motor current signal received from the inputterminal 682.

[0064] The driver sub-circuit 780 includes a comparator 802 having itsinverting input coupled to the summing junction 730 of the periodicsub-circuit 770. The non-inverting input of the comparator 802 receivesat terminal 804 the error voltage signal from the output 652 of theerror amplifier 646 of FIG. 6. An output 806 of the comparator 802 iscoupled to a Reset input of an RS flip-flop 812. The Set input offlip-flop 812 is coupled to the output 708 of the periodic sub-circuit700. The non-inverting output of the flip-flop 812 is coupled tojunction 824. NAND gate 826 receives inputs from junction 824 and Halloutput terminal 664, and has its output coupled through a resistor 814to the base of a transistor 808. An emitter of the transistor 808 iscoupled to ground potential, and a collector of the transistor 808 iscoupled at its output 810 to the input 656 of the power transistor 630of FIG. 6. NAND gate 828 receives inputs from junction 824 and Halloutput terminal 662, and has its output coupled through a resistor 815to the base of a transistor 809. An emitter of the transistor 809 iscoupled to ground potential, and a collector of the transistor 809 iscoupled at its output 811 to the input 657 of the power transistor 630of FIG. 6. The transistors 808 and 809, which serve as pwm drivertransistors, are shown as npn BJTs, but may be FETs or other suitabletransistors or other electrical conduction switches for driving thepulse width modulators of the invention. Similarly, as may be recognizedby those skilled in the pertinent art based on the teachings herein, theflip-flop 812 may take the form of any of numerous binary state or likedevices which now or later become known to those skilled in thepertinent art for performing the functions of the flip-flop describedherein.

[0065] When the output of the comparator 702 is low, the junction 714 ofthe resistors 706, 710 and 712 may be at {fraction (1/3)} V_(cc). Whenthe output of the comparator 702 is high, the junction 714 may be at{fraction (2/3)} V_(cc). The timing capacitor 718 is periodicallycharged, for example, from {fraction (1/3)} V_(cc) to {fraction (2/3)}V_(cc) by the timing resistor 716. The timing capacitor 718 isperiodically discharged from {fraction (2/3)} V_(cc) to {fraction (1/3)}V_(cc) by the timing resistor 722 through diode 720. The frequency ofoscillation is primarily a function of the capacitance level of thetiming capacitor 718 and the resistance levels of the timing resistors716 and 722. Timing resistor 716 determines the charge period, and theequivalent resistance of parallel resistors 716 and 722 determines thedischarge period. Accordingly, a ramp voltage generated by the timingresistor 716 and the timing capacitor 718 is applied to the junction 732and, in turn, to the summing junction 730. Input terminal 682 passes amotor current signal across resistor 726 to summing junction 730. Thus,the resultant signal at the summing junction 730 is approximately equalto the sum of the ramp signal and the motor current signal, and thesummed signal is coupled to the non-inverting input of the comparator802 of the driver sub-circuit 780. The output terminal 806 of thecomparator 802 is coupled to the Reset input of the flip-flop 812,thereby causing the flip-flop 812 to Reset whenever the value of themotor current plus ramp from the summing junction 730 exceeds the valueof the error signal from the input terminal 804 as applied to theinverting input of the comparator 802. The Set input of the flip-flop812, on the other hand, is activated every time the output 708 of thecomparator 702 goes low, thereby activating the inverted output offlip-flop 812 at the start of each ramp cycle coinciding with the rampsignal received at the summing junction 730 across the resistor 724.Once activated, the non-inverted output of the flip-flop 812 drives thecollector of one of the driver transistors 808 or 809 high. When thecollector of the driver transistor 808 is low, the pwm switch transistor630 of FIG. 6 is conducting to low potential. When the collector of thedriver transistor 808 is high, then the pwm switch transistor 630 isconducting from high potential. Likewise, when the collector of thedriver transistor 809 is low, the pwm switch transistor 631 of FIG. 6 isconducting to low potential. When the collector of the driver transistor809 is high, then the pwm switch transistor 631 is conducting from highpotential.

[0066] As may be recognized by those skilled in the pertinent art basedon the teachings herein, the periodic signal generator may generate anyof numerous different periodic or ramp signals suitable for performingthe functions described herein.

[0067] Referring now to the method of operation of the pwm voltageregulator circuit 600 shown in FIG. 6, the two lower MOSFETs 614 and 618are energized in response to the Hall Effect Sensor 660, which providestwo outputs 662 and 664 that are out of phase with each other. As may bereconfigured by those skilled in the pertinent art based on theteachings herein, this function also could be performed by a singleoutput Hall Switch with an inverting buffer, a Hall element withsuitable amplifier, or some other type of position sensor such as anopto/electric sensor. The purpose being to activate the associatedtransistor switch. Here, outputs are shown that are used to gate the PWMswitches 630 and 631. Alternatively, the output of the position sensorcould be used to gate the synchronous switching of lateral pairs forgreater power efficiency.

[0068] The direction of the current in the motor winding 622 isdetermined by the appropriate activation of opposing transistor pairs614 and 631, or 618 and 630. For example, when PWM Switch 630 andCommutation Switch 618 are “ON”, then one might say that the motorwinding is energized “+ to −.” When the opposite case occurs where PWMSwitch 631 and Commutation Switch 614 are “ON”, the winding would beenergized “− to +.” By repeating this sequence the motor is caused torotate, and by the reversing of the current the motor is caused to be abipolar motor.

[0069] Motor current is dropped across the resistor 680 to produce asmall voltage at terminal 682 that is used to detect the amplitude ofthe motor current. This voltage representing the motor current is to beused in the course of regulating the motor current during normal andfault conditions. The MOSFETs 614, 618, 630 and 631 are N-channel typesbut could, by appropriate circuit design, be a mixture of P-channel andN-Channel types or some other type of solid state devices such asbipolar transistors or IBGTs.

[0070] For the power circuits to function properly in this embodiment abias source of sufficient amplitude, about 10 volts higher than +Vin,must be provided to the upper side PWM switches 630 and 631. This isaccomplished with the use of bootstrap drivers comprising the bootstrapcapacitors 676 and 678, and the rectifier diodes 672 and 674. When thePWM switch 630 or 631 is “OFF”, then that PWM switch Source lead whichhas a common connection with the bootstrap capacitor 676 or 678 is atapproximately ground potential. The other end of the bootstrap capacitoris connected to −Vcc through the associated rectifier 672 or 674. Whenthe PWM switch 630 or 631 is “ON”, the PWM Switch Drain and Source areboth at nearly Vin potential. The bootstrap capacitor 676 or 678 whichwas charged to Vcc is now at Vcc+Vin. This provides the necessaryvoltage to energize the PWM Switch 630 or 631.

[0071] With reference to FIG. 7, the oscillator is composed of thecomparator 702, the resistor divider network comprising resistors 706,710 and 712, and a time constant network. The values of the resistors706, 710 and 712 determine the oscillator ramp levels at terminal 732which for a 10-volt Vcc could be around 2 volts for a ramp voltagevalley and around 7 volts for a ramp voltage peak.

[0072] The relative ratios for the charging resistor 716 and thedischarging resistor 722 could be approximately 50:1. Ramp signalfrequency is then determined primarily by charging resistor 716 andtiming capacitor 718 and could be in the area of 20 kHz. The ramp signalfrom terminal 732 is summed at summing junction 730 with the MotorCurrent Sample from terminal 682 and applied to the inverting input ofthe second comparator 802.

[0073] In order to accurately maintain motor speed over a variety ofinput voltages V_(IN) and to minimize the variation between productionmotors it is desirable to use a voltage control loop. A constant voltagedrive is believed to maintain motor speed better than a constant currentdrive over different load conditions, such as back pressure variationsor air density differences, because the motor is allowed to draw morecurrent and therefore do more or less work as required. Constant voltagedrive also results in less speed variation from motor to motor underequal loading conditions because variations from motor to motor areexpressed as differences in input current for each individual motor. Awell designed constant current drive can, however, result in lesscurrent variation within the rotational period of the motor.

[0074] By imbedding a current control loop within a voltage control loopin accordance with the invention optimal characteristics can beobtained. In other words, the voltage loop maintains a more equal speedover varying load conditions and minimizes speed deviations betweendifferent motors, and the current loop maintains a substantiallyconstant current within the rotational period.

[0075] To accomplish the voltage control the present invention comparesthe motor voltage against a reference level. This is done bydifferential amplifier 646 that has the non-inverting terminal connectedto a reference voltage. For the purpose of example that reference couldbe 0.25 volts. The motor voltage is derived with an averaging capacitor638 and two resistors 636 and 637 attached to either end of the motorwinding. The voltage on the capacitor 638 is equal to about {fraction(1/2)} of the actual motor voltage. The divider network comprised ofresistors 647 and 649 then scales that averaged voltage to be equal tothe same 0.25 volts on the inverting terminal of the differentialamplifier at the desired real motor voltage level.

[0076] The gain of the amplifier 646 is very high such that typicallyless than 1 mv of difference between inputs will result in several voltsof change in the output. This output voltage, referred to as the Errorvoltage, is in turn applied to the non-inverting input of a comparatorthrough divider resistors 820 and 822. This is to scale the full outputvoltage of the operational amplifier (646-842) to about 0.3 volts for7.5 volts on the output 806 of the amplifier 802.

[0077] The motor current sample at terminal 682, which has a level ofabout 0.2 volts for full load operation, is summed at junction 730 witha portion of the ramp obtained from the oscillator and applied to theinverting input of the comparator 802. This causes the output of thecomparator to go “LOW” when the instantaneous value of the motor sampleplus ramp exceeds the voltage level set by the Error amplifier 802.

[0078] The output of the oscillator comparator 702 is “LOW” when theoscillator timing capacitor 718 is discharged, and this is connected tothe “Set” input of flip-flop 812 so as to “Set” the flip-flop at the endof each oscillator cycle. The output of the comparator 802, whichcompares the current sample to the Error voltage, is connected to theReset input of the flip-flop 812. The “Q” output of the flip flop 812will then be “HIGH” when the oscillator starts the timing cycle, andwill go “LOW” when the current sample exceeds a level determined by thevoltage error amplifier 802.

[0079] The output or the flip-flop 812 is then applied to the inputs oftwo NAND gates 826 and 828 that also receive inputs from the Hall EffectSwitch 660. The NAND gates route the flip flop output to the appropriatedrive transistor 808-809 according to the motor position as sensed bythe Hall Effect Switch 660, which in turn enables the appropriate PWMMOSFET 630 or 631. Accordingly, the comparator 802 controls the currentin the motor by affecting interruption of current flow through the PWMMOSFETs 630 or 631. This occurs when the current exceeds a level set bythe Voltage Error amplifier 646. The amplitude of the motor current isdetermined by comparing the motor voltage against a desired value butnevertheless will remain substantially constant over the rotationperiod.

[0080]FIGS. 8A and 9A depict typical prior art pwm voltage regulatormotor input current waveforms resulting at motor speeds of about 2000and 3500 RPM respectively. As can be seen, the input waveforms arenon-symmetrical within each pulse and non-symmetrical between cycles.One advantage of the present invention is that substantially symmetricalmotor input current waveforms, such as those depicted in FIGS. 8B and9B, are attainable via application of the present invention to brushlessDC motors such as those used in the prior art.

[0081]FIGS. 10A and 11A depict typical prior art pwm voltage regulatormotor winding current waveforms resulting at motor speeds of about 2000and 3500 RPM respectively. As can be seen, the motor winding waveformsare non-symmetrical within each pulse and non-symmetrical betweencycles. Another advantage of the present invention is that symmetricalmotor winding current waveforms, such as those depicted in FIGS. 10B and11B, are attainable via application of the present invention tobrushless DC motors such as those used in the prior art.

[0082] Those skilled in the pertinent art may recognize, based on theteachings herein, that the aforementioned method of motor speedregulation may be applied to time-varying motor control with relativeease by application of a time-varying motor reference signal to thepresent invention. Accordingly, the method of the present invention mayfurther comprise the steps of controlling a pulse width of the pulsedmotor supply signal to control the average voltage level of the pulsedmotor supply signal in response to a control input via the referencevoltage corresponding to a desired change in the angular velocity of themotor.

[0083] Accordingly, although this invention has been shown and describedwith respect to exemplary embodiments thereof, it should be understoodby those skilled in the art that the foregoing and various otherchanges, omissions, and additions in the form and detail thereof may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. An apparatus for controlling a brushless DC motorhaving at least one input terminal, comprising: first means forgenerating a first signal corresponding to a motor armature voltage;second means for comparing the first signal to a reference voltage andgenerating a second signal indicative of the difference between thefirst signal and the reference voltage; third means for generating athird signal corresponding to a motor armature current; fourth means forcomparing the second signal to the third signal, and for generating afourth signal indicative of a difference between the second and thirdsignals; and fifth means coupled between the fourth means and the atleast one input terminal for pulse width modulating a power sourcesignal according to the fourth signal to thereby generate a pulsed motorsupply signal.
 2. An apparatus as defined in claim 1, wherein the thirdmeans comprises means for superimposing a ramp signal on a motorarmature current signal for generating the third signal.
 3. An apparatusas defined in claim 1, wherein the first means comprises means forsensing the motor voltage, averaging the sensed motor voltage, andgenerating the first signal based on the averaged motor voltage.
 4. Anapparatus as defined in claim 1, further comprising means for regulatingthe first time-derivative of the motor armature current of the motorsupply signal to substantially zero for the duration of each “on” pulseof the motor supply signal to thereby maintain a substantially constantmotor armature current for the duration of each pulse.
 5. An apparatusas defined in claim 1, wherein the second means comprises a differentialamplifier including a first input receiving the first signal, a secondinput receiving the reference voltage, and an output generating thesecond signal indicative of the difference between the first signal andthe reference voltage.
 6. An apparatus as defined in claim 1, whereinthe third means includes an input terminal coupled to a motor currentsource for receiving therefrom a motor armature current signal.
 7. Anapparatus as defined in claim 1, wherein the third means comprises aperiodic signal generator for generating a time-varying signal, an inputterminal coupled to a motor current source for receiving therefrom amotor armature current signal, and a summing junction coupled to theinput terminal and the periodic signal generator for summing the motorarmature current signal and the time-varying signal and generating thethird signal corresponding to motor armature current therefrom.
 8. Anapparatus as defined in claim 1, wherein the fourth means comprises acomparator including a first input for receiving the second signal, asecond input for receiving the third signal, and an output forgenerating the fourth signal indicative of a difference between thesecond and third signals.
 9. An apparatus as defined in claim 1, whereinthe fifth means comprises at least one electrical conduction switchcoupled between the fourth means and the at least one input terminalbased on the fourth signal.
 10. An apparatus as defined in claim 9,wherein the fifth means further comprises a binary state device coupledbetween the fourth means and the at least one electrical conductionswitch for pulse-width modulating said switch according to the fourthsignal.
 11. An apparatus as defined in claim 10, wherein the binarystate device includes an input coupled to a periodic signal generatorfor initiating each pulse of said switch.
 12. An apparatus as defined inclaim 11, wherein the fifth means further comprises at least one secondelectrical conduction switch coupled between at least one second inputterminal and ground potential.
 13. An apparatus as defined in claim 12,further comprising means for sensing motor winding position, and whereinthat at least the electrical conduction switch is coupled thereto.
 14. Amethod for controlling a brushless DC motor having at least one inputterminal, comprising the following steps: generating a first signalcorresponding to a motor armature voltage; comparing the first signal toa reference voltage, and generating a second signal indicative of thedifference between the first signal and the reference voltage;generating a third signal corresponding to a motor armature current;comparing the second signal to the third signal, and generating a fourthsignal indicative of a difference between the second and third signals;and pulse width modulating a power source signal according to the fourthsignal to thereby generate a pulsed motor supply signal, andtransmitting the pulsed motor supply signal to the at least one inputterminal.
 15. A method as defined in claim 14, further comprising thestep of generating a pulsed motor supply signal of substantiallysymmetrical current waveform.
 16. A method as defined in claim 14,further comprising the step of driving the at least one input terminalof the brushless DC motor with the pulsed motor supply signal toregulate the angular velocity of the brushless DC motor to thatindicated by the reference voltage.
 17. A method as defined in claim 14,further comprising the step of generating the third signal bysuperimposing a ramp signal on the signal corresponding to the motorarmature current.
 18. A method as defined in claim 14, furthercomprising the step of sensing the motor voltage, averaging the sensedmotor voltage, and generating the first signal based on the averagedmotor voltage.
 19. A method as defined in claim 14, further comprisingthe steps of generating a signal corresponding to the firsttime-derivative of the motor armature current, and regulating the signalto substantially zero for the duration of each “on” pulse of the motorsupply signal to thereby maintain a substantially constant motorarmature current for the duration of each said pulse.
 20. A method asdefined in claim 14, further comprising the step of varying the pulseduration by periodically setting a binary state device, and thenresetting the binary state device based on a comparison of motorarmature voltage error and a change in motor armature current.
 21. Amethod as defined in claim 14, further comprising the step of generatinga periodic signal and summing it with a signal corresponding to motorarmature current.
 22. A method as defined in claim 14, furthercomprising the step of generating a signal indicative of motor voltageby coupling the motor armature to a low-pass filter.
 23. A method asdefined in claim 14, further comprising the step of generating anaveraged motor voltage by integrating a voltage divider output.
 24. Amethod as defined in claim 14, further comprising the step of generatinga signal indicative of motor winding position and thereupon commutatingthe windings of a bipolar motor.
 25. An apparatus for controlling abrushless DC motor having at least one input terminal, comprising: afirst input terminal coupled to a voltage source corresponding to amotor armature voltage for generating a first input signal correspondingto motor armature voltage; a differential amplifier including a firstinput coupled to the first input terminal and receiving the first inputsignal therefrom, a second input coupled to a reference voltage sourcefor receiving a reference voltage signal therefrom, and an output forgenerating an error signal indicative of the difference between thefirst input signal and the reference voltage; a second input terminalcoupled to a motor current source for generating a second input signalcorresponding to motor armature current; a comparator having a firstinput coupled to the output of the differential amplifier for receivingthe error signal therefrom, a second input coupled to the second inputterminal for receiving the signal indicative of motor current therefrom,and an output generating an output signal indicative of a differencebetween the error signal and the motor current signal; and at least oneelectrical conduction switch coupled between the comparator and the atleast one input terminal of the motor for pulse width modulating a powersource signal according to the comparator output signal and therebygenerating a pulsed motor supply signal.
 26. An apparatus as defined inclaim 25, further comprising a periodic signal generator coupled betweenthe at least one electrical conduction switch and the second inputterminal for generating a time-varying signal and adding thetime-varying signal to the second input signal, and transmitting thesummed signal to the second input of the comparator.
 27. An apparatus asdefined in claim 26, further comprising a binary state device includingan input coupled to the output of the comparator, and an output coupledto the at least one electrical conduction switch for pulse widthmodulating said switch.
 28. An apparatus as defined in claim 27, whereinthe binary state device comprises another input terminal coupled to theperiodic signal generator for initiating each pulse of said switch. 29.An apparatus as defined in claim 28, further comprising at least onesecond electrical conduction switch coupled between at least one secondinput terminal of the motor and a second terminal of a power sourcesignal.